What kind of person would do this...What happened to self-respect?

Well, maybe she just needs to be shown love. I think she is just feeling very lonely. Who doesn't want to be loved? Unfortunately, I think your friend is equating physical relationships for love. Which simply isn't so. Maybe you need to talk to her about what is going on and explain to her what you feel and the problem may just be that she is insecure and needs someone to want her. Maybe try helping her meet someone?